1. Field of the Invention
The present invention relates to a high frequency filter for an electric instruments, and more particularly to a high frequency filter for protecting the electric instruments from inductive interference caused by various electromagnetic waves from a broadcasting station, an amateur wireless station, a civil wireless station, a personal wireless system, a military radar system and the like.
2. Description of the Prior Art
Although various high frequency filters of this kind have been proposed, reliable protection of the electric instruments from the inductive interference may not be effected due to insufficient band width of the filter characteristic. Meanwhile, a presently proposed high frequency filter effective in a broad radio frequency band is costly to manufacture. For the purpose of eliminating such problems, in copending U.S. patent application Ser. No. 755,654, filed on July 16, 1985, the inventors have proposed a high frequency filter which can be constructed at a low cost to provide a distributed constant circuit effective in a broad radio frequency band. In use in combination with various internal electronic circuit elements, however, the high frequency filter is arranged separately from the electronic circuit elements on a circuit board. In such arrangement of the high frequency filter in a casing, various wiring layers are formed on the circuit board between the high frequency filter and the electronic circuit elements. As a result, a limited space in the casing is inevitably occupied by the wiring layers.